fusego/internal/buffer/out_message.go

151 lines
4.1 KiB
Go

// Copyright 2015 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package buffer
import (
"fmt"
"log"
"reflect"
"unsafe"
"github.com/jacobsa/fuse/internal/fusekernel"
)
// OutMessageHeaderSize is the size of the leading header in every
// properly-constructed OutMessage. Reset brings the message back to this size.
const OutMessageHeaderSize = int(unsafe.Sizeof(fusekernel.OutHeader{}))
// OutMessage provides a mechanism for constructing a single contiguous fuse
// message from multiple segments, where the first segment is always a
// fusekernel.OutHeader message.
//
// Must be initialized with Reset.
type OutMessage struct {
// The offset into payload to which we're currently writing.
payloadOffset int
header fusekernel.OutHeader
payload [MaxReadSize]byte
}
// Make sure that the header and payload are contiguous.
func init() {
a := unsafe.Offsetof(OutMessage{}.header) + uintptr(OutMessageHeaderSize)
b := unsafe.Offsetof(OutMessage{}.payload)
if a != b {
log.Panicf(
"header ends at offset %d, but payload starts at offset %d",
a, b)
}
}
// Reset resets m so that it's ready to be used again. Afterward, the contents
// are solely a zeroed fusekernel.OutHeader struct.
func (m *OutMessage) Reset() {
m.payloadOffset = 0
m.header = fusekernel.OutHeader{}
}
// OutHeader returns a pointer to the header at the start of the message.
func (m *OutMessage) OutHeader() *fusekernel.OutHeader {
return &m.header
}
// Grow grows m's buffer by the given number of bytes, returning a pointer to
// the start of the new segment, which is guaranteed to be zeroed. If there is
// insufficient space, it returns nil.
func (m *OutMessage) Grow(n int) unsafe.Pointer {
p := m.GrowNoZero(n)
if p != nil {
memclr(p, uintptr(n))
}
return p
}
// GrowNoZero is equivalent to Grow, except the new segment is not zeroed. Use
// with caution!
func (m *OutMessage) GrowNoZero(n int) unsafe.Pointer {
// Will we overflow the buffer?
o := m.payloadOffset
if len(m.payload)-o < n {
return nil
}
p := unsafe.Pointer(uintptr(unsafe.Pointer(&m.payload)) + uintptr(o))
m.payloadOffset = o + n
return p
}
// ShrinkTo shrinks m to the given size. It panics if the size is greater than
// Len() or less than OutMessageHeaderSize.
func (m *OutMessage) ShrinkTo(n int) {
if n < OutMessageHeaderSize || n > m.Len() {
panic(fmt.Sprintf(
"ShrinkTo(%d) out of range (current Len: %d)",
n,
m.Len()))
}
m.payloadOffset = n - OutMessageHeaderSize
}
// Append is equivalent to growing by len(src), then copying src over the new
// segment. Int panics if there is not enough room available.
func (m *OutMessage) Append(src []byte) {
p := m.GrowNoZero(len(src))
if p == nil {
panic(fmt.Sprintf("Can't grow %d bytes", len(src)))
}
sh := (*reflect.SliceHeader)(unsafe.Pointer(&src))
memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
return
}
// AppendString is like Append, but accepts string input.
func (m *OutMessage) AppendString(src string) {
p := m.GrowNoZero(len(src))
if p == nil {
panic(fmt.Sprintf("Can't grow %d bytes", len(src)))
}
sh := (*reflect.StringHeader)(unsafe.Pointer(&src))
memmove(p, unsafe.Pointer(sh.Data), uintptr(sh.Len))
return
}
// Len returns the current size of the message, including the leading header.
func (m *OutMessage) Len() int {
return OutMessageHeaderSize + m.payloadOffset
}
// Bytes returns a reference to the current contents of the buffer, including
// the leading header.
func (m *OutMessage) Bytes() []byte {
l := m.Len()
sh := reflect.SliceHeader{
Data: uintptr(unsafe.Pointer(&m.header)),
Len: l,
Cap: l,
}
return *(*[]byte)(unsafe.Pointer(&sh))
}